The newest generation of high performance infrared imaging sensor systems employ staring focal plane detector arrays which are sensitive in both the 3-5 micron “mid-wave infrared” and 8-12 micron “long-wave infrared” spectral bands. This “dual band” operation places a great burden on refractive lens designs to maintain color aberration correction over both bands simultaneously. When the sensor is also required to provide three or four fields of view, optical complexity increases even further. Design approaches for new dual-band systems can no longer be derived from older generation single-band sensors. For example, U.S. Pat. No. 5,479,016 “Compact Second Generation FLIR Kit” (1995) describes optical systems which typify the sensor architecture for single-band high performance sensors. In general, this approach involves a re-imaging afocal assembly which projects a real pupil onto a mirror element, which in turn sends the light into a re-imager assembly which contains an intermediate focal plane and then an aperture stop located near the final focal plane. The afocal assembly includes a set of lenses which “flip in” when a wider field of view is desired, and thus the whole system is capable of providing only two fields of view in total. Furthermore, when the system is evaluated for performance in both 3-5 and 8-12 micron spectral bands, it is quickly seen that the chromatic aberrations between the two bands are very disparate and thus the basic design form can not be simply “tweaked” to achieve good performance. A survey of the current art reveals that imaging optics for dual-band systems have been developed, such as per U.S. Pat. No. 7,218,444 B2 “Dual Band, Dual Focal-Length Relayed Refractive Imager” (2007). However, these optical assemblies do not offer wide selection of field of view change options, which in fact are referenced as “optional fore-optics” in the aforementioned patent. Furthermore, the total optical system must also somehow provide high quality optical imaging of one or more thermal reference objects which are used to calibrate the detector focal plane array. This function, too, is complicated by the need for multiple field of view switching and simultaneous operation in two disparate spectral bands. In the prior art such as U.S. Pat. No. 7,218,444 B2, a typical approach to include a calibration source would involve adding a motorized flip-in mirror along with an extra set of imaging lenses adjacent to the primary optical path in order to periodically view a calibration target.